This invention relates to making thin strip and more particularly casting of thin strip by a twin roll caster.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of counter-rotating horizontal casting rolls, which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between the rolls to produce solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel, or tundish, from which it flows through a metal delivery nozzle positioned above the nip, longitudinally between the casting rolls, which delivers the molten metal to the region above the nip to form a casting pool of molten metal. The casting pool of molten metal is supported on the casting surfaces of the rolls above the nip. The casting pool is typically confined at the ends of the casting rolls by side plates or dams held in sliding engagement adjacent the ends of the casting rolls.
In casting thin strip by twin roll casting, the metal delivery nozzles receive molten metal from the moveable tundish and deposit the molten metal in the casting pool in a desired flow pattern. The flow pattern created by the manner in which the nozzle delivers molten metal to the casting pool can affect the quality and yield of the thin strip. For example, a flow pattern which causes thinning of the shells on the surface of the casting rolls before coming together at the nip is believed to cause ridges to be formed on the surface of the strip. A flow pattern which inhibits thinning of the shells on the casting roll would reduce such surface defects. Further, disturbance of the surface, or meniscus, of the casting pool has a tendency to cause meniscus marks on the surface of the strip. A flow pattern which inhibits disturbance of the surface of the casting pool is more likely to provide a metal strip with fewer meniscus marks and provide a better quality and improved yield of product.
The formation of pieces of solid metal known as “skulls” in the casting pool in the vicinity of the confining side plates or dams is a known problem. The rate of heat loss from the casting pool is higher near the side dams adjacent the casting roll ends due to the greater surface area of continuous caster components in contact with the molten metal in the casting pool increasing the conductive heat loss from the system. This area is called the “triple point region.” This localized heat loss gives rise to “skulls” of solid metal forming in that region, which can grow to considerable size. The skulls can drop through the nip of the casting rolls and into the forming strip, causing defects in the strip known as “snake eggs.” An increased flow of molten metal to the triple point regions, near the side dams, has been provided to help maintain the temperature of the casting pool in these regions. Examples of such proposals may be seen in U.S. Pat. No. 4,694,887 and in U.S. Pat. No. 5,221,511, which are both incorporated herein by reference. However, in providing increased flow in these regions it is important that the surface of the casting pool is disturbed as little as possible. Further, it is important to inhibit thinning of the shells on the surface of the casting roll in the triple point region to reduce surface defects in the strip. Also, it is important that the shells are not washed on the casting surfaces of the rolls in the triple point region, increasing the possibility of defects in the strip and reducing the quality and yield of the strip product.
The present disclosure provides a method for continuously casting metal strip, which comprises the steps of:                (a) assembling a pair of casting rolls laterally positioned to form a nip between them and to maintain a casting pool of molten metal supported by the casting rolls between side dams,        (b) assembling an elongated metal delivery nozzle extending along and above the nip, the delivery nozzle having one or more segments extending longitudinally along the metal delivery nozzle, each segment having:                    a main portion having one or more outlets positioned longitudinally along the elongated metal delivery nozzle and adapted to deliver molten metal downwardly converging toward the nip while forming the casting pool supported on the casting rolls above the nip, and            an end portion adjacent a side dam having a reservoir portion having at least one pair of passages adapted to deliver molten metal into the casting pool adjacent the side dams the flow from the at least one pair of passages converging beneath the reservoir portion,                        (c) delivering molten metal from a metal delivery system to the main portion of the segments to deliver molten metal through the one or more outlets converging downwardly toward the nip, while forming the casting pool of molten metal supported on the casting rolls above the nip, and through the at least one pair of passages in the reservoir portion in the end portions into the casting pool adjacent the side dams, and        (d) counter-rotating the casting rolls to form shells on the casting surfaces of the casting rolls brought together at the nip to cast metal strip downwardly from the nip.        
Also disclosed is an apparatus for continuously casting metal strip, comprising:                (a) a pair of casting rolls laterally positioned to form a nip between them and side dams to form a molten metal pool supported by the casting rolls between side dams and adapted to counter rotate to form shells on the casting rolls brought together at the nip to cast metal strip downwardly from the nip;        (b) an elongated metal delivery nozzle extending along and above the nip, the delivery nozzle having one or more segments each having:                    a main portion extending along the elongated metal delivery nozzle with one or more outlets positioned longitudinally along the elongated metal delivery nozzle and directed converging downwardly toward the nip while forming a casting pool of molten metal supported on the casting rolls above the nip, and            an end portion adjacent side dams having a reservoir portion having at least one pair of passages adapted to deliver molten metal into a molten metal pool adjacent the side dams with flow from the at least one pair of passages converging below the reservoir portion; and,                        (c) a metal delivery system adapted to introduce molten metal through the segments of the elongated metal delivery nozzle downwardly converging toward the nip.        
In some alternatives of the above method and apparatus, the end portion of each segment has a laterally extending weir adapted to allow molten metal to flow over the weir between the reservoir portion and the main portion. Optionally in addition, or in the alternative, the main portion of each segment extends beneath the reservoir portion into the end portion.
In other alternatives of the above method and apparatus, the outlets in the main portion of each segment are in a pair of rows of outlets and deliver molten metal with flow from each row of outlets converging toward flow from the outlets of the other row. The pair of rows of outlets in the main portion of each segment may be angled such that their directions of flow converge below the delivery nozzle. In other alternatives, the outlets in the main portion of each segment may be configured to flow downwardly at an angle between 5 and 60 degrees substantially centered about a vertical centerline through the elongated delivery nozzle. Additionally, the at least one pair of passages in the reservoir portion of each segment may be angled such that their directions of flow converge below the reservoir portion. The passages of the at least one pair of passages in the reservoir portion of each segment may be positioned between 40 and 160 millimeters apart, or, alternatively, between 50 and 125 millimeters apart. Furthermore, the at least one pair of passages in the reservoir portion may have an angle of convergence between 5 and 60 degrees substantially centered about a vertical centerline through the elongated delivery nozzle.
In further alternatives of the above method and apparatus, the one or more outlets may be a channel extending longitudinally along each segment. The channel may have substantially parallel sides, or, in the alternative, the channel may have tapered sides.
The delivery nozzle of the above method and apparatus may further comprise a restrictive baffle in the main portion adapted to cause the molten metal to flow laterally within the delivery nozzle. In other alternatives the baffle may be adapted to support a pool of molten metal in the main portion of the delivery nozzle. The baffle may be adapted to reduce the velocity of the molten metal passing through the delivery nozzle, and optionally may have a convex or concave portion adapted to reduce the velocity, or change the direction, of the molten metal passing through the delivery nozzle.
The main portion of the delivery nozzle may comprise one or more passages above the one or more outlets, adapted to deliver molten metal to the outlets, and, optionally, the bottom portion of the main portion of the delivery nozzle may be tapered toward the one or more passages to converge the molten metal flow toward the nip between casting rolls of a twin roll caster. Alternatively, the bottom portion of the main portion of the delivery nozzle may be tapered toward the one or more outlets to converge the molten metal flow toward the nip between casting rolls.
Also disclosed is a method of continuously casting metal strip comprising:                (a) assembling a pair of casting rolls laterally positioned to form a nip between them and adapted to maintain a casting pool of molten metal supported by the casting rolls adjacent side dams,        (b) assembling an elongated metal delivery nozzle extending along and above the nip, the delivery nozzle having:                    at least one segment having a main portion, the main portion having one or more outlets adapted to deliver molten metal in the casting pool longitudinally along the metal delivery nozzle directed downwardly toward the nip; and,            a restrictive baffle, positioned above the outlets adapted to alter the velocity of the molten metal flowing through the main portion of delivery nozzle and tapered sidewalls and/or passages below the baffle enables molten metal to flow below the baffle to converge toward the nip,                        (c) introducing molten metal from a metal delivery system through the elongated metal delivery nozzle downwardly toward the nip forming a casting pool of molten metal supported on the casting rolls above the nip, and        (d) counter-rotating the casting rolls so as to form shells on the casting surfaces of the casting rolls brought together at the nip to cast metal strip downwardly from the nip.        
Additionally disclosed is an apparatus for continuously casting metal strip comprising:                (a) a pair of casting rolls laterally positioned to form a nip between them and adapted to maintain a casting pool of molten metal supported by the casting rolls between side dams- and adapted to counter rotate to form shells on the casting rolls brought together at the nip to cast metal strip downwardly from the nip,        (b) an elongated metal delivery nozzle extending along and above the nip, the delivery nozzle having:                    at least one segment having a main portion, the main portion having one or more outlets positioned along the delivery nozzle adapted to deliver molten metal in the casting pool longitudinally along the metal delivery nozzle directed downwardly converging toward the nip; and            a restrictive baffle positioned above the outlets adapted to alter the velocity of the molten metal flowing through the main portion of the delivery nozzle and downwardly in the delivery nozzle; and,                        (c) a metal delivery system for introducing molten metal through the segments of the elongated metal delivery nozzle downwardly converging toward the nip.        
In some embodiments of the above method and apparatus, the elongated delivery nozzle has segments positioned end to end adapted to deliver molten metal in the casting pool along the metal delivery nozzle directed downwardly converging toward the nip.
The elongated metal delivery nozzle of the above method and apparatus for continuously casting metal strip may also comprise an end portion adjacent side dams having a reservoir portion having at least one pair of passages adapted to deliver molten metal into a molten metal pool adjacent side dams, the directions of flow to converge below the reservoir portion. The entries of each passage of the at least one pair of passages in the reservoir portion may be positioned between 40 and 160 millimeters apart, or between 55 and 125 millimeters apart, and may have an angle of convergence between 5 and 60 degrees substantially centered about a vertical centerline through the elongated delivery nozzle. Optionally in addition, or in the alternative, the reservoir portion in the end portion of each segment may have a laterally extending weir adapted to allow molten metal to flow over the weirs between the main portion and the reservoir portion. The main portion may extend beneath the reservoir portion into the end portion of each segment.
The baffle may comprise one or more passages adapted to allow molten metal to flow through the passages. The baffle may be adapted to cause the molten metal to flow laterally within the main portion of the delivery nozzle. In addition, or in the alternative, the baffle may be adapted to reduce the velocity of the molten metal flowing through the elongated delivery nozzle. The baffle may be adapted such that a pool of molten metal is formed in the main portion of the delivery nozzle above the baffle. Further, in some alternatives, the baffle may be removable from each segment of the elongated delivery nozzle.
In some embodiments, the one or more outlets in the main portion of the elongated metal delivery nozzle may be one or more pairs of rows of outlets positioned longitudinally along the elongated metal delivery nozzle directed downwardly toward the nip such that the directions of flow of the pair of outlets converge below the delivery nozzle. The at least one pair of outlets may be angled such that their directions of flow converge below the reservoir portion. The angle of convergence of the outlets may be between 5 and 60 degrees substantially centered about a vertical centerline through the elongated delivery nozzle.
In some alternatives, the one or more outlets in the main portion of each segment is at least one channel extending longitudinally along each segment. The channel may have substantially parallel sides, or, in the alternative, the channel may have tapered sides. The main portion may further comprise one or more passages above the channel, adapted to deliver molten metal to the channel. In embodiments, the one or more passages may be positioned below the baffle. In addition, or in the alternative, the main portion may be tapered toward the one or more passages. In addition, or in the alternative, the bottom portion of the main portion may be tapered toward the one or more outlets.
Various aspects of the invention will be apparent from the following detailed description, drawings and claims.